1. Field of the Invention
The present invention relates to a decision circuit having a transistor-differential amplifier, and more particularly, to a decision circuit operable at a wide range of input and reference voltages.
2. Description of the Related Art
A decision circuit or comparator typically compares an input voltage with a reference voltage and judges whether or not the input voltage is higher than the reference voltage. A conventional decision circuit or comparator, capable of operating at a low driving voltage, for example, at a Vcc of about 1 V, comprises an NPN transistor-differential amplifier. In order for the transistor of the differential amplifier to be turned on, an input voltage applied to the transistor must at least be equal to the required base-to-emitter turn-on voltage (i.e., about 0.7 V) of the transistor, and must also allow for the resulting base-to-collector voltage of the transistor to be met. Hence, the operative input voltage range for this differential amplifier is from about 0.7 V (i.e., the base-to-emitter turn-on voltage) to about 1 V (i.e., the supply voltage Vcc). Generally, a conventional comparator such as this is inoperable at a range of input voltages under the base-to-emitter voltage (e.g., 0.6 or 0.7 V), when the supply voltage is 1.0 V.
When it is desirable to lower the detection-voltage range of the decision circuit, the NPN transistor can be replaced with a PNP transistor. The detection-voltage range is an input voltage range at which the transistor is operative as a comparator. In this case, the operative input-voltage range is from ground (i.e., 0 V) to about (Vcc -0.7) V, (i.e., 0.3 V).
A technique for changing between the NPN and PNP differential amplifiers is described,.for example, in Japanese Laid-Open Patent Application No. 198815/1989 (JP-A-01-198815). However, such comparators have a detection-voltage range which is so limited, that they will not operate when an input voltage as small as about one third (e.g., 1/3 V) or two thirds (e.g., 2/3 V) of the supply voltage (e.g., 1 V) is applied. Hence, this substantially limits the operability of a decision circuit, such as a portable decision circuit driven by a single battery cell, which comprises these types of comparators and is required to be operated at a low driving voltage.